112 ADJUSTMENT TO ENVIRONMENT 



of the stimulus of gravity, toward the wet soil in the ex- 

 posed bottom of the vessel. Though the experiment is 

 less easy, it can nevertheless be shown that roots will 

 turn away from a poor soil toward a fertile one. This 

 sort of response is known as Chemotropism. 



Thermotropism is the response to differences of tem- 

 perature. To some sorts of plants this is important. 

 For example, in the cacti of arid regions it keeps the roots 

 near the surface of the soil where they can best take ad- 

 vantage of the scanty rainfall when it does come. Under 

 experimental conditions their roots may be induced to 

 turn down and penetrate deeply into the soil if it is heated 

 from below. 



Other Tropisms of Roots. — Roots are also sensitive 

 to the direction of light. This is useful in some cases, 

 particularly in roots which originate on parts of a plant 

 exposed to light. Thus, on the English ivy, roots origi- 

 nate on the dark side of stems next the wall (Fig. 23) 

 on which it is growing, where they can secure a hold and 

 fasten the shoot securely in place. 



It can be shown experimentally that roots are sensitive 

 to the direction of a current of water. They will also 

 respond to an electric current and to still other sorts of 

 stimuli. It seems probable, however, that most of these 

 are of little or no consequence in nature. They are men- 

 tioned here merely to call attention to the fact that proto- 

 plasm often has unutilized possibilities but which might 

 conceivably sometime be of use to it under new condi- 

 tions. This fact should be borne in mind when reading 

 the chapter on Evolution in reference to the question 

 whether the environment really produces the character- 

 istics of organisms or whether the characteristics they 

 already have determine whether and where they can live. 



The Adjustment of Stems is primarily controlled by 

 responses to light and gravity (Figs. 24, 25). Unlike 

 roots, they grow away from the earth and toward the 

 light. In a very young seedling, even where the stem 



